Conventionally, various kinds of electric devices provided to the vehicle are supplied with power with a battery and an alternator as the power source. When the vehicle starts from idle stop, the power to the electric devices is supplied by the battery. When this is done, if the output voltage of the battery becomes lower than the driving voltages of the electric devices because of cranking, a trouble occurs such that the memory of the navigation system is gone. Therefore, when the vehicle starts from idle stop, the output voltage of the battery is raised before supplied to the electric devices such as the navigation system necessarily supplied with power of not less than a predetermined voltage at all times (for example, see Japanese Patent Application Laid-Open No. 2004-92564).
On the contrary, when the engine is being driven, since the output voltage of the alternator is higher than the driving voltages of the electric devices, power is supplied to the electric devices with the output voltage of the alternator being lowered, thereby reducing power consumption. For example, Japanese Patent Application Laid-Open No. H10-266920 describes a vehicle fuel pump motor control circuit that improves fuel economy by finely PWM-controlling the voltage applied to the motor of the fuel pump.